Radio-frequency (RF) including microwave frequency transponders are used for contactless identification, remote location and tracking of physical objects to which the transponders are fixed. Such transponders are used in supermarkets, for warehouse inventories, on vehicles and on personnel. Typically, each transponder has a unique identification code which is modulated and transmitted on an RF carrier or microwave frequency. The transponder is interrogated by a reader which transmits an RF signal to the transponder. The transponder responds by returning its identification code to the reader.
So-called passive transponders often employ transmitters powered by the radio frequency or microwave signal received from the transponder reader. Power conversion circuitry is provided in the transponder but is often restricted by the performance of the diodes and other components therein. As a result, the range of so-called passive transponders (such as the transponder shown in FIG. 1 of the accompanying drawings) is severely limited, especially in the microwave frequency range of above 1 gigahertz.
Such transponders usually transmit over a wide undirected radiation pattern and thus the power of the signal from the transponder is dissipated quickly. This serves to reduce the range of operation between the transponder and reader. Retrodirective or Van Atta arrays enhance the range of such transponder reader systems by correcting the phase of the wavefront impinging on the transponder and redirecting the wave back to the reader in the same direction from which the wave was transmitted.
As show in FIGS. 2 and 3 of the accompanying drawings, respective pairs of antenna elements are connected by equal lengths of co-axial cable connection lines to form a Van Atta array. However, such Van Atta arrays are large and unwieldy due to the constraints imparted on the system because of the equal connection line lengths between antenna elements.
Various methods of improving the retrodirectivity of transponders have been put forward and are disclosed in, for example, U.S. Pat. No. 3,496,570 which uses a slab of dielectric to cancel specularly reflective waves and U.S. Pat. No. 3,731,313 which uses impedance matching stubs attached to respective connection lines.
U.S. Pat. No. 5,254,997 discloses an RF transponder incorporating a Van Atta array. However, the transponder disclosed therein operates by receiving the first signal, modulating the first signal with an identification code and then reflecting the modulated signal back to an interrogator. Such a transponder is shown schematically in FIG. 4 of the accompanying drawings.